Material for the controlled vaporization of a liquid cryogen

ABSTRACT

A shaped article is capable of at least one of containing and delivering a cryogenic fluid. The article has a porous structure that restricts the passage of cryogenic fluid in the liquid phase while permitting the passage of cryogenic fluid in the gaseous phase. The article may be in the form of a tube or container. The article permits a liquid cryogen to be transported to a specific site, and then cool the site by means of conduction from the cold article and convection of cold gas, the phase change of the evaporating liquid greatly enhancing the heat loss.

RELATED APPLICATIONS

This application is a divisional of application Ser. No. 09/327,808,filed Jun. 8, 1999 now U.S. Pat. No. 6,427,451.

FIELD OF THE INVENTION

The present invention relates to a material used to facilitate thedelivery and controlled evaporation of a liquid cryogen. Shaped articlesof the present invention are capable of containing and delivering acryogenic fluid. These articles have a porous structure that restrictsthe passage of cryogenic fluid in the liquid phase while permitting thepassage of cryogenic fluid in the gaseous phase. Such fluids may includenitrogen, helium, hydrogen, argon, neon and air as well as liquefiedpetroleum gas or low temperature liquids.

By “restrict” or “restriction” in this context is meant that while gascan exit a material of the present invention through its exteriorsurface, liquid will enter into the thickness of the material but willnot pass as a liquid through its exterior surface under specificoperating conditions (e.g., temperature, humidity, pressure, etc.).

By “low temperature” in this context is meant a temperaturesubstantially below 0° C. Typically liquid nitrogen, for example, isliquid at temperature of approximately 77 Kelvin (−196° C.) at anatmospheric pressure of one atmosphere.

BACKGROUND OF THE INVENTION

Two primary technologies are used for the transport or storage of coldliquids or liquids with a low heat of vaporisation, namely, thoseutilising vacuum insulation and those that operate by dry gas retention.Unlike articles of the present invention, neither of these technologiescontrols the release of gaseous cryogenic fluid through the exteriorsurface of the container or conduit.

U.S. Pat. No. 5,511,542 (Westinghouse Electric Corporation) discloses agarment incorporating a conduit constituted by, for example, a Dacron®tube surrounded by a sheath of non-woven cotton. The conduit is statedto be impermeable to liquids but permeable to gases. A conduit of thisnature is unlike conduits in accordance with embodiments of the presentinvention. Cryogenic liquids enter the structure of conduits of thepresent invention and at high enough pressures liquid cryogens leakthrough the conduit walls. At pressures lower than those that causeliquid leakage through the walls, cryogenic fluid in the gaseous phaseexits the exterior surface of the conduit as evidenced by a plume ofwater condensate.

Cooling garments, such as the Cooling Suit supplied by Aerospace Designand Development, Inc. Niwot, Colo., as part of the SCAMP® (supercriticalair mobility pack) model number 547-000-06 require the use of a coolantthat primarily remains in a liquid phase. These garments require a fluidcontrol and heat exchange system, which is heavy. In addition to theextra weight to be carried, such a system has the significantdisadvantages of high purchase and service costs. Cooling garments ofthe present invention possess advantages over cooling garments in theprior art. These advantages include lower weight, lower volumes ofliquid coolant used, simpler system control requirements and no need forpumps or fans and their associated power and control requirements.

Various polymers are known to be useful under low temperature conditionssuch as 77 Kelvin. For example, porous polytetrafluoroethylene (PTFE) isknown to retain strength and flexibility at low temperatures,particularly in the form of porous expanded PTFE (ePTFE) constituted bynodes interconnected by fibrils as described in U.S. Pat. No. 3,953,566to Gore. Such ePTFE, however, is not normally suitable for the transportor storage of cryogenic liquids because of its porosity, which allowscryogenic liquids to have ready passage into and through the ePTFEmaterial.

Temperature gradients affecting materials used in systems such as thoseinvolving cryogens are such that thermal expansion and contractioneffects cause early mechanical failure in components. Preferredembodiments of this invention, in addition to possessing certainpermeation characteristics, relate to materials that retain flexibilityand strength at low temperatures, typically 77 Kelvin.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided amaterial for the transport of a cryogenic fluid, said material having aporous structure which allows a liquid cryogenic fluid to enter througha first surface of the material into the thickness of the material butrestricts leakage of liquid cryogenic fluid through the exterior, orsecond, surface. The first and second surfaces are separated by thethickness. The restriction may occur within the thickness of thematerial and/or at the exterior surface at the first and/or or interiorsurface. Furthermore, the material preferably also controls passage ofthe cryogenic fluid in gaseous phase through the exterior surface of thematerial.

In its preferred form, the invention provides a liquid permeationrestriction material that preferably is lightweight and flexible at lowtemperatures. It allows evaporative cooling using liquid cryogenicfluids, which affords more efficient cooling than by simply transportingand delivering a gaseous cryogenic fluid. Articles formed of material ofembodiments of the present invention afford the ability to transport aliquid cryogen to a specific site, then cool that site by means ofconduction from the cold material and convection of a cold gas. The heatloss is greatly enhanced by the phase change of the evaporating liquid.

According to a further aspect of the present invention there is provideda garment incorporating a conduit of a material with permeationqualities as set out in the preceding paragraphs.

Preferably, the material of the present invention is in the form of atube.

Preferably also, a plurality of layers of material are superimposed oneach other to provide a multi-layered composite material possessing aspiral-shaped cross-section, formed from one or more sheets of film.Furthermore, a tube possessing a spiral-shaped cross-section may becomprised of more than one type of film.

The porous material of the invention results in a product whichpreferably has a high restriction to the through flow of liquid throughthe wall of the material whilst having a low content of solid material.This preferred material provides improved mechanical and permeationcharacteristics particularly when used in a multi-layered construction.A multi-layered construction may result in an article that exhibits lowbending stresses, thereby increasing its fatigue life. The summation ofseveral layers of material may also increase the pressure required toforce liquid cryogen through to the exterior surface.

The material of the present invention may be utilised to restrict liquidcryogen permeation through the material to a rate that will facilitateheat loss through liquid to vapour phase change within the material andat the external surface of the material.

Cryogenic fluid permeation articles made from material of the presentinvention enable the passage of the gaseous phase of cryogenic fluidsacross the thickness direction of the article, while inhibiting thepassage of the liquid phase of the fluids across the thicknessdirection. In these articles, the mass flow rate of the liquid phase ofa cryogenic fluid flowing through the wall in the thickness direction isless than or equal to the mass evaporation rate of the liquid at theouter wall surface. The material may be modified to alter therestriction of liquid phase cryogenic fluid passage and the controlledrelease of gaseous phase cryogenic fluid through the exterior of thematerial. A preferred article in the form of a cryogenic fluidpermeation tube has a liquid nitrogen leak pressure (LNLP) (based on thetest described below) of at least 0.3 psi (0.002 MPa). Such a tubeperforms satisfactorily in a cryogenic cooling garment, tested in amanner described below. The tube does not leak liquid nitrogen duringthe 15 minute test duration. Tubes preferred for use in a cryogeniccooling garment possess a LNLP of at least 0.3 psi (0.002 MPa) and donot fracture during flexure at cryogenic temperatures. Tubes havinghigher values for LNLP and that do not fracture at these temperaturesare more preferred for use in this application; a more preferable tubefor use in a cooling garment possesses a liquid nitrogen leak pressure(LNLP) such as 0.45 psi (0.003 MPa).

Any suitable porous material may be used, including polymers, metals,ceramics and mixtures or composites thereof. Fluoropolymer is consideredsuitable and porous expanded PTFE (ePTFE) is a particularly preferredmaterial because of its flexibility at cryogenic temperatures and theability to fabricate a tube and other forms from ePTFE with a desiredpermeability. Although ePTFE is not brittle at very low temperatures,care must be taken in the construction of tubes, and other forms, toensure that the structure or density of the final tube does not lead tofracture at these temperatures. Non-porous tubes not only typicallypossess extremely poor permeation properties, they also tend to beunacceptably stiff and prone to fracture, especially at cryogenictemperatures. Low porosity tubes also appear prone to fracture atcryogenic temperatures.

PTFE-based articles of embodiments of the present invention are alsopreferred because of the low thermal conductivity of PTFE, which isabout 0.232 Watts/m.K. Porous articles of PFTE exhibit even lowerthermal conductivity. The use of low thermal conductivity materials mayresult in safer articles with regard to issues such as potential forcold burns. Cryogenic fluid systems will benefit from lower thermalenergy ingress and resulting reduction in gas generation within thefluid transport lines. PTFE additionally has a low heat capacity,1047kJ/kg K.

The choice of precursor ePTFE film material is a function of the desirednumber of layers in the final tube, tube wall thickness, airpermeability and pore size of the final tube. Pore size may be assessedby isopropanol bubble points (IBP) measurements. Films possessing highIBP values appear to produce final tubes with higher values for LNLP.The use of smaller pore size films appears to increase the LNLP of thefinal tube. Increased number of layers and increased film thickness mayalso increase the LNLP of the final tube. The number of layers ispreferably between 8 and 48, more preferably between 12 and 24. The LNLPis preferably between 0.003 and 0.075 MPa, more preferably between 0.04and 0.06 MPa. An ePTFE base tube may also be part of the construction,but the inclusion of a base tube appears not to be critically important.A suitable tube has been constructed using a porous ePTFE filmpossessing a thickness of 0.0035 inch (0.09 mm), a 39.5 Gurley numberand 48.5 psi (0.334 MPa) IBP.

Externally applied reinforcement in the form of rings or helicallyapplied beading or filament or other configurations or materials may beincorporated into the tube construction in order to provide kink and/orcompression resistance to the article.

An article in accordance with an aspect of the invention in the form ofa membrane suitable for allowing the passage of the gaseous phase of acryogenic fluid while restricting the passage of the liquid phase of thesame cryogen may be produced by a similar process. Multiple layers offilm may be wrapped onto a large diameter mandrel, the ends restrainedand the assembly placed in an oven in order to bond the layers togetherusing the films and process temperatures described in the examplesbelow. The large diameter tube thus produced may be slit longitudinallyto provide a flat membrane. Other techniques may be employed to bondfilm layers to produce a membrane. The resultant membrane may be used tocreate more complex shapes, such as pouches, flat constructions withpredefined conduits therein or as a liner for storage tanks.

Other articles made from material of the present invention may be usefulfor cooling warm objects, such as electronic devices, engines, motors,heated elements, and so forth.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic perspective view illustrating a first method ofproducing an article in accordance with an embodiment of the presentinvention, said article being in the form of a tube;

FIG. 2 is a diagrammatic perspective view illustrating a second methodof producing an article in accordance with an embodiment of the presentinvention, said article being in the form of a tube;

FIG. 3 is a cross-sectional view of a tubular article in accordance withone embodiment of the present invention;

FIG. 4 is a cross-sectional view of a tubular article in accordance withanother embodiment of the present invention;

FIG. 5a is a diagrammatic representation of an article in accordancewith yet another embodiment of the present invention, said article beingin the form of a pouch;

FIG. 5b is a half-sectional representation of the pouch of FIG. 5a;

FIG. 5c is an enlarged view of area 5 c of FIG. 5b;

FIG. 6 is a diagrammatic representation of an article in accordance withyet another embodiment of the present invention, said article being inthe form of a membrane having channels incorporated therein fortransporting a cryogenic fluid;

FIGS. 7 and 8 are diagrammatic representations of a garment intended tobe worn in an environment where cooling of the wearer is desirable, saidgarment incorporating a tubular conduit in accordance with an embodimentof the present invention;

FIG. 9 is a diagrammatic illustration of one form of test apparatus fortesting the efficiency of tubular articles in accordance withembodiments of the present invention;

FIGS. 10a and 10 b are diagrammatic representations of a garment used inthe Cryogenic Cooling Garment Test described hereinafter;

FIG. 11 is a scanning electron micrograph (SEM) referred to in Example 1as described hereinafter;

FIG. 12 is a diagrammatic illustration of test apparatus that is amodified version of the apparatus illustrated in FIG. 9 for testing theefficiency of articles in accordance with embodiments of the presentinvention;

FIG. 13a is a cross-sectional view of an article in accordance withanother embodiment of the present invention, said article being in theform of a tube with a helically-applied reinforcement;

FIG. 13b is a cross-sectional view taken on line 13 b—13 b of FIG. 13a;

FIG. 14a is a cross-sectional view of a tubular article in accordancewith another embodiment of the present invention, said articlecontaining more than one type of film material;

FIG. 14b is an enlarged sectional view taken on line 14 b—14 bof FIG.14a;

FIG. 15a is cross-sectional view of a tubular article in accordance withanother embodiment of the present invention, said article constructedfrom a film comprising more than one material;

FIG. 15b is an enlarged sectional view taken on line 15 b—15 b of FIG.15a;

FIGS. 16a, 16 b and 16 c are diagrammatic illustrations of the use ofarticles of the present invention for cooling electronic devices; and

FIG. 17 is a diagrammatic illustration of another form of test apparatusfor testing the efficiency of (membrane) articles in accordance withembodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, FIG. 1 illustrates a first method ofproducing a tubular article from the material of an embodiment of theinvention. In this method, one or more layers of film 10, such as porousexpanded polytetrafluoroethylene (ePTFE) film, is or are helicallywrapped around a mandrel 11. The ends of the tube 1 2 thus formed aresecured and the assembly is subjected to temperatures above thecrystalline melt point of PTFE. The tube 12 should be sufficientlystrong in the longitudinal direction to enable its removal from themandrel 11 without suffering damage. Helical wrapping in two directionsmay impart different properties to the tube.

FIG. 2 illustrates a second method of producing a tubular article frommaterial of an embodiment of the invention. The method is the same asthat for the tube 12 of FIG. 1 except that the wrapping is not carriedout in a helical fashion but is effected by circumferential wrapping ofa long length of porous film (such as ePTFE) film 13 about thelongitudinal axis of a mandrel 14 to form a tube 15. Either thelongitudinal or transverse direction of the film 13 may be wrapped ontothe mandrel 14. Circumferential wrapping of long length film 13 suchthat the film is wrapped directly from the takeoff of a film spool on tothe mandrel 14 limits the final tube length to the width of theprecursor film 13.

The wrapping techniques described with reference to FIGS. 1 and 2 allproduce a tube possessing a spiral cross-section 16 as shown in FIG. 3.

If desired the tubes 12 and 15 of FIGS. 1 to 3 may be provided with aporous base tube 17 as shown in FIG. 4. In the finished tube of FIG. 4,the base tube 17 forms a luminal surface. In the FIG. 4 embodiment, thebase tube 17 is applied to the mandrel before one or more layers offilm, such as porous ePTFE is or are wrapped around the exterior surfaceof the base tube 17.

In any of the embodiments of FIGS. 1 to 4, the finished tube may beconstituted by layers comprising a combination of both helical andcircumferential wrapping.

Although the inventive article, such as a tube as described withreference to FIGS. 1 to 4, may be constituted by a single sheet ofporous film it may be preferred that the articles of the invention,including the tubes of FIGS. 1 to 4, are constituted by multiple sheetsof porous film.

When producing a multi-layered article, such as a tube as in FIGS. 1 to4, the multi-layered film assembly is heated at sufficient temperatureand a long enough time to ensure bonding of the layers. Insufficientheating may result in a tube prone to delamination. The number of filmlayers may be varied in order to optimise tube wall thickness and tubeflexibility. The diameter of the mandrel may be varied to produce a tubeof a desired inner diameter.

Although the embodiments of FIGS. 1 to 4 are in the form of tubes, itwill be readily apparent to those of skill in the art that articles inaccordance with embodiments of the present invention may take formsother than tubular. For example, a pouch 18 of porous ePTFE may beformed as shown in FIG. 5. Alternatively, the porous material may formother containers in a variety of shapes, conduits, container liners,membranes or the like which are intended to facilitate the containmentduring transport or storage of a low temperature, low surface energyfluid such as cryogenic liquid.

In order to produce a membrane suitable for forming the pouch 18 asshown in FIG. 5, multiple layers of film are wrapped onto a largediameter mandrel, the ends restrained and the assembly placed in an ovenin order to bond the layers together using the films and processtemperatures described in the examples below. The large diameter tubethus produced is slit longitudinally to provide a flat membrane and theresultant membrane formed into a pouch 18, the multi-layered nature ofthe membrane 19 being evident from FIGS. 5b and 5 c. Of course such amembrane may be formed into other shapes and forms, such as a flatconstruction 20 with predetermined conduits 21, as illustrated in FIG.6.

FIGS. 7 and 8 illustrate a particular embodiment of the presentinvention in which a conduit in the form of a tube of porous ePTFEcapable of containing a cryogenic fluid such as liquid nitrogen, argon,or liquid air and which will allow the gaseous phase of the fluid topermeate to the exterior of the tube is incorporated in a protectivegarment such as may be worn by a firefighter or the like.

FIGS. 7 and 8 are respectively front and back views of a fire-fightinggarment 22. The garment 22 incorporates a container 23 for containingliquid nitrogen or liquid air (in this example reference will be made toliquid nitrogen) connected to distribution tubes 24 forming a network oftubes for distributing the liquid nitrogen throughout the garment. Thesystem of tubes 24 is located between an insulation layer of the garmentand an inner lining.

The container 23 for holding liquid nitrogen comprises an insulatedpressure vessel for holding the liquid nitrogen and a valve mechanism 25controlled by a valve-trigger 26 for allowing passage of liquid nitrogeninto the tubes 24. The tubes 24 are connected to the valve-triggermechanism 26 via a restriction orifice, the restriction of whichdetermines the cooling rate. The valve-trigger 26 allows the flow to beturned on and off or to be regulated. The liquid nitrogen container 23contains a 0.5 kg charge which lasts for approximately 35 minutes atfull gas delivery. Over this time period, 0.5 kg of liquid nitrogenprovides approximately 100 watts of cooling. The container 23 is of asuitable shape to be located in a pocket inside or more preferablyoutside the garment where it may be manually controlled by the wearer.

When the liquid nitrogen is fed into the network of tubes 24, thenitrogen permeates through the wall of the tubes to emerge from theouter surface of the tubes in gaseous phase. The evaporative transitionof the nitrogen from the liquid to gaseous phase provides a coolingeffect at the outer tube surface which is transmitted to the wearer ofthe garment.

In an alternative embodiment, it is possible for the flow to beregulated by an electronic control means responsive to temperatureswithin the garment, so that the garment temperature is maintainedthermostatically to a constant value.

Liquid Nitrogen Leak Pressure Test

A liquid nitrogen leak pressure test was developed to measure thepressure at which liquid nitrogen permeates through a cryogen tube wall.Liquid nitrogen is added to the lumen of tested tubes and pressurised.The tube is examined to ensure the permeation of gaseous nitrogenthrough the tube wall. The pressure at which liquid nitrogen leaksthrough the walls of the tube is noted and recorded. This pressurecorresponds to the pressure at which the mass flow rate of liquidnitrogen flowing through the wall in the radial direction exceeds themass evaporation rate of the liquid at the outer wall surface. Aschematic representation of the test apparatus appears in FIG. 9. A 0.5Liter Dewar flask (Cryo Jem. Cryomedical Instruments Ltd.Nottinghamshire UK) 30 is obtained (a larger flask may be used ifdesired.) The Dewar flask lid 31 is dried to avoid the outlet valve 35becoming blocked due to moisture ingress leading to accumulation of iceparticles. The Dewar flask 30 is filled with liquid nitrogen and the lid31 slowly screwed onto the canister allowing excess liquid nitrogen toboil off.

Air pressure is applied to the top of the liquid nitrogen reservoir. Thepressure is regulated via a precision regulator (Moore. Model 41-100)32. A pressure monitoring tap is included in the line entering the flaskfor safety reasons. The Dewar flask 30 inlet pressure is measured with amulti-port pressure transducer (Heise, model PM. Newtown. Conn.) orgauge 33. Liquid nitrogen is forced out of the flask through a 0.062inch (1.58 mm) inner diameter stainless steel dip tube 34 that extendsfrom near the bottom of the flask to an opening in the flask lid 31. Alever valve 35 at the head controls the exit flow. The dip tube 34extends beyond this valve 35, enclosed in a larger plastic conduit 36.Threaded fittings 37 are attached to the larger conduit 36. Anotherpressure monitoring tap is included in the line in order to measure theinlet pressure to the tested tube (using the same pressure monitor asdescribed above or guage 38). A standard barb fitting 40 is screwed intothe fitting 37.

The tube 39 to be tested is cut to a length of 180 mm. The test lengthis about 135 mm since portions of the ends are attached over fittings40, 42. One end of the tube 39 is attached over the barb fitting 40 andsecured by wrapping silver plated copper wire 41 tightly around theoutside of the tube 39. The other end of the tube 39 is fitted with abarb fitting 42 and secured in the same manner. The outlet of this barb42 fitting is fitted with a 0.50 inch (12.7 mm) long PTFE cylindricalplug 43. The plug 43 has a 0.062 inch (1.58 mm) diameter, 0.075 inch(1.90 mm) long hole 44 drilled through its centre, which iscounter-bored to 0.125 inch (3.18 mm) diameter for a length of 0.425inch (10.8 mm). The outlet orifice diameter and dip tube inside diameterare specified to match. These are the smallest flow restrictions in theline exiting the flask. This choice of outlet orifice 44 and dip tubeinside diameter enables a sufficient test duration before exhausting theliquid nitrogen from the flask. Venting the outlet to atmosphereenhances the flow of liquid nitrogen into the tube to be tested.

The tube 39 is positioned horizontally. The test is performed under ahood at ambient conditions: room temperature is 19.6° C., relativehumidity is about 46% and in essentially still air. The nitrogen exitingthe end of the tube is directed outside of the hood in order not todisturb the air flow under the hood.

The tube 39 is tested in the following manner. The Dewar flask levervalve 35 is opened. The pressure regulator 32 is adjusted until liquidnitrogen exits the orifice 44 at the end of the test sample tube. Thedischarge of liquid nitrogen is readily confirmed by placing an expandedPTFE membrane in the path of the exiting nitrogen and noting wetting ofthe membrane. All fittings and connection are examined to ensure that noleaks are present. The tube 39 is then examined for gaseous permeationof nitrogen through its wall, along the length of the tube as evidencedby a plume of condensed water vapour in the vicinity of the tube. Theapplied pressure is adjusted until such a steady plume is observed. Asteady plume indicates both gas permeation and that the air is still inthe test environment. The plume as described demonstrates that gaseousnitrogen is exiting along the length of the tube 39, which is indicativeof distributed evaporative cooling. Note that the pressure increase inthe Dewar flask 30 resulting from the evaporation of the nitrogen alonemay be sufficient to pressurise the tube 39.

The tube under test is allowed to chill for a period of 30 seconds priorto further pressure adjustment. The pressure is increased until thefirst droplet of liquid nitrogen appears on the outer surface of thetested tube 39. The pressure regulator 32 is slowly and slightly openedand closed to ensure that this is the pressure corresponding to theformation of the first stable droplet. A stable droplet is one thatunder constant pressure, remains about the same size during testing forat least 5 seconds, without dripping. By decreasing the pressure thedroplet will evaporate. With increasing pressure, the droplet sizeincreases past stability until liquid is first dripping rapidly and thenrunning out of the tube wall. The pressure measured at the entrance tothe tested tube 39 is recorded. This average of three pressure readings,taken at intervals of at least 20 seconds as measured with the pressuregauge 38 is recorded as the liquid nitrogen leak pressure in Table 2.Venting the tube 39 to atmosphere via the use of the plug 43 with the0.062 inch (1.58 mm) orifice 44 is important to achieve the distributionof liquid nitrogen across the length of the tube 39. Tubes in accordancewith the preferred embodiments of the present invention permeate themost gas when liquid cryogen is present on the interior surface. Boilingof the liquid inside the tube appears to enhance gaseous permeation.

Whereas this test was developed specifically for testing tubes, the sameprinciples may be applied to create a test for the examination of theproperties of other shapes of materials. The important elements of thetest include: controlled application of pressure and ability to measurethe pressure required to force a mass of liquid nitrogen sufficient toform a stable drop of liquid on the outside wall of the test article,through the thickness of the article while the internal surface of thearticle is in contact with liquid.

A liquid nitrogen leak pressure test can also be performed on a flatsheet article of the present invention. A schematic representation ofthe test apparatus appears in FIG. 17. A cylinder 100 is equipped with afill lid 101 and pressure relief valve 102, a pressurization means 103,and a pressure measurement guage 107. The flat sheet article 104 isattached to the bottom of the cylinder by a ring 105 and clamps 106. Thecylinder is filled with liquid nitrogen to cover the sheet sample andthe lid 101 is screwed on slowly to allow excess nitrogen to boil off.Air pressure is applied to the top of the cylinder and is regulated andmonitored as previously described for the liquid nitrogen leak pressuretesting of tubes. The test is also conducted as previously described.During these tests the article under test must be exposed to the sameenvironmental temperature and humidity conditions as stated previously,allowing stable convection and evaporation conditions at the outersurface of the test article.

Cryogenic Cooling Garment Test

A tube 45 is placed inside a vest and connected at one end to a Dewarflask 47 containing liquid nitrogen as indicated in FIGS. 10a and 10 b.The other end of the tube 45 is vented to atmosphere. A subject wearsthe vest over a shirt and wears a fire jacket over the vest. The subjectwalks on a treadmill set at a rate of 3 miles (4.8 km)/hour at a 5%incline. The test is conducted in a room at ambient conditions: roomtemperature is 21° C., relative humidity is about 41% and essentiallystill air. The cooling system is worn over underwear and under a heavy,insulated jacket, minimum weight 1.5 kg, during test.

Isopropanol Bubble Point, Gurley Air Permeability and Tube DimensionMeasurement Testing for the Tubes

The tubes are mounted to barbed luer fittings and secured with clampsand tested intact. The values of three samples per tube are obtained andaveraged for the isopropanol (IPA) bubble point and the thicknessmeasurements. One Gurley air permeability measurement is made per tube.

The isopropanol bubble points (IBP) are tested by first soaking thetubing fixtures in IPA for approximately six hours under vacuum, thenremoving the tubing from the IPA and connecting the tubing to an airpressure source. Air pressure is then manually increased at a slow rateuntil the first steady stream of bubbles is detected. The correspondingpressure is recorded as the IBP.

The air permeability measurement is determined using a Gurley Densometer(Model 4110, W. & L. E. Gurley, Troy, N.Y.) fitted with an adapter platethat allows the testing of a length of tubing. A one foot length oftubing is tested, unless otherwise noted. The average internal surfacearea is calculated from the measurements utilising a Ram OpticalInstrument (OMIS II 6×12, Ram Optical Instrumentation Inc., 15192 TritonLane, Huntington Beach, Calif.). The Gurley Densometer measures the timeit takes for 100 cc of air to pass through the wall of the tube under4.88 inches (12.40 cm) of water head of pressure. The air permeabilityvalue is calculated as the inverse of the product of the Gurley numberand the internal surface area of the tube expressed in units of cc/mincm².

The wall thickness and inner diameter of the tube are measured using thesame OMIS II optical system.

Bubble Point and Thickness Testing for Films

Bubble point of films is measured according to the procedures of ASTMF31 6-86. The film is wetted with isopropanol or methanol, as noted inthe examples.

Film thickness is measured with a snap gauge (Mitutoyo, model 2804-10,Japan).

Flexibility Test

The tube is placed in a 1.5 L Dewar flask filled with liquid nitrogenfor a period of 30 seconds. The tube is removed and quickly wrappedaround a hollow steel cylinder having an outer diameter of 1.5 inch(38.1 mm) and a wall thickness of 0.05 inch (1.27 mm). The tube isvisually examined for evidence of fracture, to determine if the wrappinghad compromised the ability of the tube to hold liquid (liquid argon isused in testing the tubes of Example 8). A tube that does not fractureduring this test is considered to be flexible.

EXAMPLE 1

Expanded PTFE film is obtained in a 42 inch (106.7 mm) width possessinga thickness of 0.0035 inch (0.09 mm), a Gurley number of 39.5 secondsand an isopropanol bubble point of 48.5 psi (0.334 MPa). Allmeasurements are made in accordance with the procedures previouslydescribed, unless otherwise indicated. The film is circumferentiallywrapped around a 0.25 inch (6.4 mm) stainless steel mandrel such thatthe width of the film becomes the length of the resultant tube asdepicted in FIG. 2. Twelve layers of film are wrapped around themandrel. The cross-sectional geometry of the layered construction isspiral-shaped as indicated in FIGS. 3 and 4. The construction parametersfor this and other examples appear in Table 1.

The ends of the layered film construction are restrained by suitablemeans to prevent shrinkage in the longitudinal direction of theconstruction (the longitudinal axis of the mandrel) during subsequentheat treatment.

TABLE 1 Example Tube Length Wrap type Number of Layers Base tube MandrelID Salt Bath 1 106.7 cm circum. 12 no 6.4 mm 365 deg C./1.5 min 2 106.7cm circum. 12 yes 6.4 mm 365 deg C./2 min   3 106.7 cm circum. 12 yes6.4 mm 365 deg C./2 min   4 109.2 cm helix 18 yes 6.4 mm 366 deg C./2min   5  86.4 cm longitudinal 12 no 6.4 mm 366 deg C./1.5 min 6 at leastcircum./helix 8 circum./48 helical yes 4.0 mm not applicable 109.8 cm 8a 96.5 cm circum. 18 no 6.4 mm 362 deg C./2.5 min  8b  95.2 cm circum. 24no 6.4 mm 362 deg C./2.5 min ID = inner diameter

The restrained construction is submerged in a 365° C. molten salt bathoven for 1.5 minutes in order to bond the ePTFE layers and impartdimensional stability to the tube. The tube is allowed to cool thenwashed in ambient temperature water to remove residual salt. The clampsare removed and the tube is removed over the end of the mandrel.

The tube is measured for inner diameter, wall thickness, Gurley number,and IBP in accordance with the techniques previously described. The tubeis also tested to determine if it serves as an effective conduit for thetransport of liquid nitrogen while allowing the passage of gaseousnitrogen through the wall. Further tests are performed to determine thepressure at which the tube passes liquid nitrogen through the wall. Thetest results for this and other examples appear in Table 2. The tubecontrols the passage of gaseous nitrogen and inhibits the passage ofliquid nitrogen at an average LNLP of 6.0 psi (0.041 MPa). Theindividual pressure readings are 5.8 psi (0.040 MPa), 5.8 psi (0.040MPa) and 6.4 psi (0.044 MPa).

A portion of the tube is dipped in liquid nitrogen for 30 seconds thenquickly wrapped around the outside of a 1.5 inch (38.1 mm) outerdiameter, 0.05 inch (1.27 mm) wall thickness steel hollow cylinder todemonstrate the flexibility of the cold tube. The tube does not fractureunder these conditions.

A scanning electron micrograph of the tube cross-section (a view takentransverse from the longitudinal axis of the tube) appears in FIG. 11. A10 micrometer reference bar appears at the bottom right of the figure.

This tube is tested as a cooling garment tube as described above. A 36inch length of this tube is used to create a cryogenic cooling garmentas illustrated in FIG. 10. The subject walks on the treadmill whilewearing the garment. The tube and garment perform satisfactorily. Thetube does not leak liquid nitrogen and permeates enough gaseous nitrogento keep the subject cool throughout the test.

This tube is also tested to measure the flow rate of gaseous nitrogenpermeating through the wall. The test set up described for measuring theliquid nitrogen leak pressure is slightly modified from that of FIG. 9,and is illustrated in FIG. 12. The change consists of enclosing the tube50 inside a cylindrical enclosure 52 of about 1.5 inch (38.1 mm) innerdiameter such that the tube 50 still vents to atmosphere. All of the gaspermeating

TABLE 2 Example Tube ID Tube Thickness Tube Gurley** Tube IBP LNLP AirPermeability 1 5.78 mm 0.56 mm  77.4 0.385 MPa 0.041 MPa 1.39 cc/min cm²2 6.04 mm 0.56 mm  82.1 0.387 MPa 0.003 MPa/0.005 MPa 1.26 cc/min cm² 36.24 mm 1.14 mm  75.1 0.259 MPa 0.046 MPa 1.34 cc/min cm² 4 6.36 mm 0.67mm 185.4 0.420 MPa 0.075 MPa 0.54 cc/min cm² 5 5.79 mm 0.61 mm  82.00.365 MPa 0.057 MPa 1.32 cc/min cm² 6 3.91 mm 0.33 mm  49.3 0.185 MPa0.003 MPa 6.50 cc/min cm² 8a 6.33 mm 64.7 mm 714.8 0.446 MPa *0.035 Mpa 0.35 cc/min cm²  8b 6.25 mm 66.8 mm 477.2 0.460 MPa *0.049 Mpa  0.44cc/min cm² *these values represent the liquid argon leak pressure **inunits of: seconds per 100 cc of air at 4.88 inches (12.40 cm) of waterID = inner diameter

through the wall of the tube 50, however, is contained within theenclosure 52. An air flow meter (range 2-20 standard cubic feet per hour(scfh) [0.06-0.6 standard cubic meters per hour (scmh)], King InstrumentCo.) 54 is connected to a port in the wall of the enclosure 52. The flowrate of the permeating gas is measured. At a pressure of 2.5 psi (0.017MPa) as indicated by pressure transducer or guage, the flow rate of gasthrough the wall of this tube 50 is measured within 2.5 minutes. Liquidnitrogen does not leak through the tube wall at this pressure. Flowrates of 3.5 scfh (0.10 scmh), 3.7 scfh (0.11 scmh) and 4.0 scfh (0.12scmh) result. Note that the measurements are not corrected fortemperature or for the use of nitrogen gas.

EXAMPLE 2

An additional tube is made in accordance with the same steps andmaterials as described in Example I and Table I except for thedifferences noted as follows.

A thin longitudinally expanded PTFE tube is obtained possessing a wallthickness of 0.119 mm, an inner diameter of 3.0 mm, and an IBP of 1.0psi (0.007 MPa). This tube is snugly slipped over the 0.25 inch (6.4 mm)diameter mandrel. The ePTFE film of Example 1 is then applied over thethin ePTFE base tube in the same manner as the film is applied to themandrel in Example 1. The construction is restrained then heated in a365° C. molten salt bath for 2 minutes, cooled, washed in ambient water,then removed from the mandrel. In all examples, the presence of a basetube results in easier removal of the tube from the mandrel.

The tube is tested as described in Example 1 and the results appear inTable 2. The tube controls the passage of gaseous nitrogen and inhibitsthe passage of liquid nitrogen at an average LNLP of 0.4 psi (0.003MPa). The three individual pressure readings are all 0.4 psi. Anotherportion of the same tube is tested. All three liquid nitrogen leakpressures are 0.7 psi (0.005 MPa). A portion of the tube is dipped inliquid nitrogen for 30 seconds then quickly wrapped around the outsideof a 1.5 inch (38.1 mm) outer diameter, 0.05 inch (1.27 mm) wallthickness steel hollow cylinder to demonstrate the flexibility of thecold tube. The tube does not fracture under these conditions.

EXAMPLE 3

Another tube is created in this same manner as Example 2 except that ⅛inch (3.18 mm) PTFE dry paste-extruded beading is applied in a helicalfashion to the base tube prior to the application of the film. Thebeading is applied with a ¾ inch (19.05 mm) lead. The purpose of thebeading is to impart greater compression resistance and kink resistanceto the final tube upon bending.

An example of a tube including such beading is illustrated in FIGS. 13aand 13 b, although in the illustrated tube the beading 57 is providedbetween two wrapped films 58 and 59, rather than between a base tube anda wrapped film.

The tube is tested as described in Example 1 and the results appear inTable 2. The tube controls the passage of gaseous nitrogen and inhibitsthe passage of liquid nitrogen at an average LNLP of 6.6 psi (0.046MPa). The individual pressure readings are all 6.6 psi (0.046 MPa). Aportion of the tube is dipped in liquid nitrogen for 30 seconds thenquickly wrapped around the outside of a 1.5 inch (38.1 mm) outerdiameter, 0.05 inch (1.27 mm) wall thickness steel hollow cylinder todemonstrate the flexibility of the cold tube. The tube does not fractureunder these conditions.

EXAMPLE 4

A tube is made using the film of Example 1. The film is slit to providea width of 0.875-inch (22.2 mm) and is helically applied over the basetube of Example 2. The film is applied with approximately 50% overlap toprovide about 18 layers of film over the base tube. The restrainedconstruction is placed in a 366° C. molten salt bath for 2 minutes. Thetube enables the passage of gaseous nitrogen and inhibits the passage ofliquid nitrogen at an average LNLP of 10.9 psi (0.075 MPa). Theindividual pressure readings are 9.0 psi (0.062 MPa), 9.0 psi (0.062MPa) and 14.8 psi (0.102 MPa). A portion of the tube is dipped in liquidnitrogen for 30 seconds then quickly wrapped around the outside of a 1.5inch (38.1 mm) outer diameter, 0.05 inch (1.27 mm) wall thickness steelhollow cylinder to demonstrate the flexibility of the cold tube. Thetube does not fracture under these conditions.

EXAMPLE 5

A tube is made in accordance with the same steps and materials asdescribed in Example I and Table 1 except for the differences noted asfollows.

The film of Example 1 is circumferentially wrapped around a 0.25 inch(6.4 mm) stainless steel mandrel such that the length of the filmbecomes the length of the resultant tube. As in Example 1, twelve layersof film are wrapped around the mandrel. This method of constructionenables the creation of a length of tube that is not limited to thewidth of the film. The restrained construction is submerged in a 366° C.molten salt bath oven for 1.5 minutes in order to bond the ePTFE layersand impart dimensional stability to the tube.

The tube controls the passage of gaseous nitrogen and inhibits thepassage of liquid nitrogen at an average LNLP of 8.2 psi (0.057 MPa).The individual pressure readings are 8.2 psi (0.057 MPa), 8.2 psi (0.057MPa) and 8.3 psi (0.057 MPa).

A portion of the tube is dipped in liquid nitrogen for 30 seconds thenquickly wrapped around the outside of a 1.5 inch (38.1 mm) outerdiameter, 0.05 inch (1.27 mm) wall thickness steel hollow cylinder todemonstrate the flexibility of the cold tube. The tube does not fractureunder these conditions.

EXAMPLE 6

A tube is made from three components, combining the construction methodsof Examples 1 and 4. That is, a base tube is placed over a 4.0 mm outerdiameter mandrel, followed by circumferentially wrapping a film over thebase tube, and finally wrapping yet another film helically atop thecircumferential layers. The base tube is a longitudinally expanded PTFEtube possessing a wall thickness of about 0.410 mm, an inner diameter of3.9 mm, and an IBP of 1.1 psi (0.008 MPa). The circumferentially wrappedfilm is an expanded PTFE film approximately 0.0017 inch (0.04 mm) thick,having an IBP of about 29.1 psi (0.201 MPa) and a Gurley number of about17.7 sec. Eight layers of this film are applied such that the transversedirection (width) of the film is oriented in the longitudinal directionof the mandrel.

Another type of film is next applied to the construction. This film is afluorinated ethylene propylene-coated porous ePTFE film. This film ismade by a process that comprises the steps of:

a) contacting an ePTFE film with a layer of fluorinated ethylenepropylene (FEP);

b) heating the composition obtained in step a) to a temperature abovethe melting point of the FEP;

c) stretching the heated composition of step b) while maintaining thetemperature above the melting point of FEP; and

d) cooling the product of step c). In this case, the FEP adhesivecoating on the porous expanded PTFE film is discontinuous (porous) dueto the amount and rate of stretching, the temperature during stretching,and the thickness of the FEP adhesive prior to stretching.

This film has an MBP of 1.7 psi (0.012 MPa) and a thickness of 0.0004inch (0.01 mm). The MBP is measured in the same manner as is IBP forfilm, except that methanol is substituted for isopropanol. This film isslit to a 0.5 inch (12.7 mm) width and then applied helically inmultiple traverse passes up and down the length of the mandrel at angles15° off perpendicular in order to apply 48 layers.

The restrained construction is placed in a convection oven set at 380°C. for 4.9 minutes in order to bond the ePTFE layers and impartdimensional stability to the tube. The tube controls the passage ofgaseous nitrogen and inhibits the passage of liquid nitrogen at anaverage LNLP of 0.4 psi (0.003 MPa). The individual pressure readingsare 0.3 psi (0.002 MPa), 0.3 psi (0.002 MPa) and 0.4 psi (0.003 MPa).The length of sample used for the isopropanol bubble point and Gurleyair permeability testing is 6.0 inch (15.2 cm).

A portion of the tube is dipped in liquid nitrogen for 30 seconds thenquickly wrapped around the outside of a 1.5 inch (38.1 mm) outerdiameter, 0.05 inch (1.27 mm) wall thickness steel hollow cylinder todemonstrate the flexibility of the cold tube. The tube does not fractureunder these conditions.

This tube is tested as a cooling garment tube as described above. A43.25 inch (109.8 cm) length of this tube is used to create a cryogeniccooling garment. The subject walks on the treadmill while wearing thegarment. The tube and garment perform satisfactorily. The tube does notleak liquid nitrogen and permeates enough gaseous nitrogen to keep thesubject cool throughout the test.

EXAMPLE 7

A commercially available rigid ceramic tube was obtained (FERRO CeramicWFAO-NAJADE (800), Rochester, N.Y.) and tested. The tube dimensions aremeasured using a digital calliper. The inner and outer diameters are14.8 mm and 22.1 mm, respectively. The tube is tested for LNLP asdescribed above. The test cannot be performed as required because thetube leaks prior to increasing the pressure enough to enable liquidnitrogen to exit the downstream orifice. Therefore, a value for LNLPcannot be obtained. A plume of gaseous nitrogen along the length of thetube, absent liquid nitrogen leakage through the exterior surface, doesresult at low pressures, namely at 0.2 psi (0.001 MPa).

EXAMPLE 8

Two tubes are made in accordance with the process described in Example1, except as noted in Table 1. The tubes are made with a differentnumber of layers and placed in a molten salt bath set at a differenttemperature, for a different period of time as compared to the tube ofExample 1. The tubes are tested for leak pressure in the mannerdescribed above except that argon is used as the cryogenic liquidinstead of nitrogen. The results appear in Table 2.

(a) the 18 layer tube controls the passage of gaseous argon and inhibitsthe passage of liquid argon at an average leak pressure of 5.1 psi(0.035 MPa). The individual pressure readings are 5.4 psi (0.037 MPa),5.1 psi (0.035 MPa) and 4.9 psi (0.034 MPa). A portion of the tube isdipped in liquid nitrogen for 30 seconds then quickly wrapped around theoutside of a 1.5 inch (38.1 mm) outer diameter, 0.05 inch (1.27 mm) wallthickness steel hollow cylinder to demonstrate the flexibility of thecold tube. The tube does not fracture under these conditions.

(b) the 24 layer tube controls the passage of gaseous argon and inhibitsthe passage of liquid argon at an average leak pressure of 7.1 psi(0.049 MPa). The individual pressure readings are all 7.1 psi (0.049MPa). A portion of the tube is dipped in liquid nitrogen for 30 secondsthen quickly wrapped around the outside of a 1.5 inch (38.1 mm) outerdiameter, 0.05 inch (1.27 mm) wall thickness steel hollow cylinder todemonstrate the flexibility of the cold tube. The tube does not fractureunder these conditions.

The length of samples used for the isopropanol bubble point and Gurleyair permeability testing is 4.8 inch (12.2 cm) and 5.7 inch (14.5 cm)for Examples 8a and 8b, respectively.

Those of skill in the art will realise that other constructions andforms of tubes may be produced, as illustrated in FIGS. 14 and 15. FIGS.14a and 14 b illustrate a tube 60 which has been formed by wrapping twosheets 62, 64 of different material around a mandrel. FIGS. 15a and 15 billustrate a tube 70 which has been formed by wrapping a sheet ofmaterial 72 around a mandrel. The sheet of material 72 is comprised oftwo materials 74, 76 bonded together, Either or both materials 74, 76may be adhesive materials. Tubes may also be constructed from two ormore sheet materials wrapped together around a mandrel. These sheetmaterials may or may not be bonded together.

Further, tubes formed in accordance with embodiments of the inventionmay be used in a wide variety of transport, storage and coolingapplications, and a number of possible cooling arrangements areillustrated in FIGS. 16a, 16 b and 16 c. In FIG. 16a, a tube 80 is shownpassing around an individual component 82 mounted on a PCB 84.Alternatively, as illustrated in FIG. 16b, a tube 84 may be arranged topass around a PCB 86 carrying a number of components 88. FIG. 16cillustrates a sheet 90, similar to that shown in FIG. 6, wrapped arounda PCB 92, with conduits 94 formed between appropriate membrane sheets.In each of these arrangements, liquid nitrogen, liquid air, or anothercryogenic fluid is passed through the tubes in liquid form, the tubesproviding cooling by conduction from the “cold” tubes and by convectionby the cold gas evaporating at or from the walls of the tubes.

Of course similar arrangements may be utilised in cooling other objects,including parts of the human body, engines, motors, electricalconductors and the like. Cooling arrangements may be provided for use byworkers experiencing elevated temperatures in the course of their work,such as fire-fighters, miners working deep underground, operators insteelworks, racing car drivers and the like. Such cooling arrangementsmay also be of assistance to workers who must wear heavy or warmprotective clothing. The consequential flow of gaseous fluid around thewearer's body may also assist in preventing or minimising the build-upof perspiration beneath the wearer's clothes, which may be waterproof orof a construction or arrangement which limits circulation of air. Inthese or other circumstances the arrangement may utilise liquid air andan arrangement for ensuring a supply of air reaches the wearer and thusprovides a supply of cool air for breathing. Such cooling arrangementsmay also be of assistance in medical or veterinary applications, forexample where it is useful for a patient's body, or part of a patient'sbody, to be cooled. In other applications, cooling sheets or enclosuresmay be utilised to facilitate storage of foodstuffs and othertemperature sensitive supplies.

Articles may be produced in accordance with the present invention with awide variety of possible designs and properties to suit particularapplications. For example, specific design modifications that may becontemplated within the scope of the present invention include:providing a conduit that has various permeabilities along its length(for instance, regions ranging from permitting no liquid entry into thematerial to regions permitting liquid leakage through the exteriorsurface of the material); having conduits that have variouspermeabilities around their circumference, so that gas leakage occursonly at pre-determined places around the circumference; having modifiedsegments along the conduit (e.g., being wider or narrower or havingmodified shapes, etc.) to provide specific delivery properties; etc.

While particular embodiments of the present invention have beenillustrated and described herein, the present invention should not belimited to such illustrations and descriptions. It should be apparentthat changes and modifications may be incorporated and embodied as partof the present invention within the scope of the following claims.

What is claimed is:
 1. A garment incorporating a shaped article, saidarticle comprising a polymer having a porous structure that restrictsthe passage of cryogenic fluid in the liquid phase while permitting thepassage of cryogenic fluid in the gaseous phase.
 2. The garment of claim1 wherein the article comprises a material that has a thermalconductivity of about 0.1 Watts/m K or lower.
 3. A garment incorporatinga shaped article, said article having a porous structure and a liquidnitrogen leak pressure of greater than or equal to about 0.3 psi.
 4. Thegarment of claim 3 wherein the article is flexible below 0° Celsius. 5.The garment of claim 4, wherein said article is flexible at cryogenictemperatures.
 6. The garment of claim 5, wherein said article isflexible at about 77 Kelvin.
 7. The garment of claim 4 wherein saidarticle is comprised of a polymer.
 8. The garment of claim 7 wherein thepolymer is polytetrafluoroethylene.
 9. The garment of claim 7 whereinthe polymer is expanded polytetrafluoroethylene.
 10. The garment ofclaim 7 in which said article comprises multiple layers of PTFE film.11. The garment of claim 3 wherein the article is in the form of a tube.12. The garment of claim 11 wherein said tube is comprised of a polymer.13. The garment of claim 12 wherein the polymer is PTFE.
 14. The garmentof claim 12 wherein the polymer is expanded polytetrafluoroethylene. 15.The garment of claim 7 in which said article is comprised of PTFE and atleast one other polymer.
 16. The garment of claim 15 in which the otherpolymer is a copolymer of hexafluoropropylene and tetrafluoroethylene.17. The garment of claim 15 in which the other polymer is a copolymer oftetrafluoroethylene and perfluoropropylvinyl ether.
 18. The garment ofclaim 11 in which said tube is comprised of PTFE and at least one otherpolymer.
 19. The garment of claim 18 in which the other polymer is acopolymer of tetrafluoroethylene and hexafluoropropylene.
 20. Thegarment of claim 18 in which the other polymer is a copolymer oftetrafluoroethylene and perfluoropropylvinyl ether.
 21. The garment ofclaim 11 in which said tube comprises multiple layers of PTFE film. 22.The garment of claim 11 wherein the tube further includes externalreinforcement.
 23. The garment of claim 3 wherein the article comprisesa material that has a thermal conductivity of about 0.23 Watts/mK orlower.
 24. The garment of claim 3 wherein said article has a LNLP ofgreater than or equal to about 0.45 psi.
 25. The garment of claim 24wherein said article has an LNLP between 0.003 and 0.075 Mpa.
 26. Thegarment of claim 25 wherein said article has an LNLP between 0.04 and0.06 Mpa.
 27. A garment comprising: a conduit comprising a polymermaterial for carrying liquid cryogenic fluid into the interior of thegarment; and connection means for connecting the conduit to a source ofcryogenic fluid.
 28. A garment according to claim 27 wherein the conduitcomprises a material having a thickness and pore structure which permitsthe flow of cryogenic fluid through its thickness such that liquidcryogenic fluid enters one face of the material and gaseous cryogenicfluid exits the other face of the material.
 29. The garment of claim 28wherein the polymer is expanded polytetrafluoroethylene.
 30. A garmentaccording to claim 28 wherein the garment is in the form of a vest. 31.A garment of according to claim 28 wherein the garment is a waterproofgarment.
 32. A garment according to claim 28 wherein the garment is awhole body covering garment.
 33. A garment according to claim 28 whereinthe garment is a water resistant water vapour permeable garment.
 34. Thegarment of claim 28 wherein the conduit includes external reinforcement.35. A garment according to claim 27 wherein the polymer is afluoropolymer.
 36. The garment of claim 35 wherein the polymer ispolytetrafluoroethylene.
 37. The garment of claim 27 in which saidconduit is comprised of PTFE and at least one other polymer.
 38. Thegarment of claim 37 in which the other polymer is a copolymer ofhexafluoropropylene and tetrafluoroethylene.
 39. The garment of claim 37in which the other polymer is a copolymer of tetrafluoroethylene andperfluoropropylvinyl ether.
 40. The garment of claim 37 in which saidconduit comprises multiple layers of PTFE film.